An aircraft is propelled by several turbojets each housed in a nacelle also accommodating a set of associated actuation devices linked to its operation, such as a thrust reverser device, and performing various functions when the turbojet is in operation or stopped.
A nacelle usually has a tubular structure comprising an air intake upstream of the turbojet, a middle section designed to surround a fan of the turbojet, a downstream section accommodating thrust reversal means and designed to surround the combustion chamber of the turbojet, and is usually terminated by an exhaust nozzle the outlet of which is situated downstream of the turbojet.
Modern nacelles are designed to accommodate a bypass turbojet capable of generating, by means of the blades of the fan in rotation, a flow of hot air (also called the main flow) originating from the turbojet combustion chamber and a flow of cold air (the bypass flow) which flows outside the turbojet through an annular passageway, also called a stream, formed between a fairing of the turbojet (or an internal structure of the downstream structure of the nacelle and surrounding the turbojet) and an internal wall of the nacelle. The two air flows are discharged from the turbojet through the rear of the nacelle.
Each propulsion assembly of the aircraft is therefore formed by a nacelle and a turbojet, and is suspended from a fixed structure of the aircraft, for example beneath a wing or on the fuselage, by means of a pylon, or mast, attached to the turbojet in its front and rear portion by suspension elements.
In such a configuration, it is the turbojet that supports the nacelle.
Such an architecture is subjected to many combined external forces during the mission of the aircraft. They are, amongst others, resultant forces of gravity, external and internal aerodynamic forces, gusts of wind and thermal effects.
These stresses applied to the propulsion assembly are transmitted to the turbojet and cause deformations of casings which have a direct impact on the efficiency of the various stages of the turbojet. More particularly, in the case of a propulsion assembly called wasp-waisted, that is to say having a downstream portion that is long and thinner than the intermediate structures and air intake, these stresses result in a deformation that is particularly harmful called “banana-shaping”, the downstream portion curving considerably.
Such “banana shaping” is reflected by a deformation of the external structure of the nacelle formed by the various successive casings while the drive shaft, the fan blades and the blades inside the turbojet remain rectilinear. The result of this is that the blade tips of the shaft move closer to the internal periphery of the casings. The general performance of the turbojet is thereby reduced relative to a configuration in which the casings sustain very little deformation or none at all, because account must then be taken of this deformation in the design of the nacelle so as to always arrange a sufficient clearance between the blade tips and the periphery of the casings. This results in a portion of the supply air not being compressed by the blades because it escapes through this considerable clearance.
The French Patent application that is not yet published and registered under number 06.05912 proposes a solution consisting in supporting the propulsion assembly by means of an internal fixed structure of the nacelle (known as the IFS).